In recent years, with the development of digital technologies, electronic devices such as portable information devices and information home appliances have further been sophisticated. As these electronic devices are sophisticated, nonvolatile memories used in the devices are rapidly developed to achieve size increase, higher integration, and higher speed. Furthermore, applications of the nonvolatile memories are dramatically increased.
An example of these applications is a memory in which nonvolatile variable resistance elements serving as memory elements are arranged in a matrix. The memory is expected to be a three-dimensional memory with still further size increase, higher integration, and higher speed.
Each of the nonvolatile variable resistance elements has a thin layer (film) made mainly of a metal oxide. By pulsing the thin layer, an electrical resistance value of the thin layer is changed and the thin layer keeps the resulting electrical resistance value. Therefore, if binary data represent a high resistance state and a low resistance state of the thin layer, for example, “1” represents the high resistance state and “0” represents the low resistance state, it is possible to write such binary data into the variable resistance element. Here, (a) a current density caused by pulsing the thin layer in the variable resistance element and (b) a size of an electrical field produced by the pulsing are set to be enough to change the physical state of the thin layer without damaging it.
Such a variable resistance element expressing binary values is classified into (a) a so-called unipolar variable resistance element having a resistance value that is changed by pulsing with one polarity and different voltages, or (b) a so-called bipolar variable resistance element having a resistance value that is changed by pulsing with different polarities. In general, the unipolar variable resistance element has characteristics of having a longer writing time in a so-called resetting process for changing the variable resistance element from a low resistance state to a high resistance state, than in a so-called setting process for changing the variable resistance element from a high resistance state to a low resistance state. On the other hand, the bipolar variable resistance element has a short writing time both in the setting process and in the resetting process.
In a so-called crosspoint memory, variable resistance elements are arranged on respective crosspoints between word lines and bit lines. The word lines and bit lines are at right angles to each other without contact. The crosspoint memory sometimes has a trouble (hereinafter, referred to as “write disturb”) when data is written to a selected target variable resistance element. In the write didturb, sneak current is occurred to change electrical resistance values of other non-selected variable resistance elements. Therefore, it is necessary to provide a special structure in the cross point memory in order to prevent write didturb.
In the unipolar variable resistance element, resistance of the variable resistance element is changed by pulsing with one polarity. Therefore, write didturb can be prevented by arranging unipolar current steering elements in series in the variable resistance element. Examples of the unipolar current steering elements are p-n junction diodes and Schottky diodes. The unipolar current steering elements have non-linear voltage-current characteristics in which the unipolar current steering elements have a high resistance state and a low resistance state at a voltage ranging in the same polarity. In other words, the unipolar current steering elements have non-linear voltage-current characteristics which allow data to be read from or write into a selected target unipolar variable resistance element in a certain voltage range having voltage-current characteristics of a low resistance state.
There have been disclosed a memory that is capable of preventing write didturb, including series circuits in each of which a variable resistance element and a Schottky diode (current steering element) are connected in series with each other (see Patent Literature 1, for example).
Regarding the disclosed memory, the Schottky diode prevents sneak current from flowing into the variable resistance element in each of memory elements except a selected target memory element to which data is to be written. As a result, the crosspoint memory can prevent write didturb. Here, in the memory disclosed in Patent Literature 1, data is written to a target variable resistance element by pulsing with one polarity to the target variable resistance element. Therefore, the Schottky diode connected in series with the target variable resistance element prevents disturbance on the data writing.
On the other hand, regarding the bipolar variable resistance elements, a bipolar electrical pulse is applied to a target variable resistance element in order to write data into the target element. Therefore, it is a bipolar current steering element to be arranged in series with the target variable resistance element. The bipolar current steering elements have non-linear voltage-current characteristics in which the bipolar current steering elements have a high resistance state in a voltage range of a positive polarity and a low resistance state in a voltage range of a negative polarity. In general, the bipolar current steering elements is in a high resistance state in a voltage range where an absolute value of an applied voltage is smaller than a predetermined threshold value, and is in a low resistance state in a voltage range where the absolute value exceeds the threshold value. Examples of the bipolar current steering elements having such characteristics are two-terminal elements such as a Metal-Insulator-Metal (MIM) diode, a Metal-Semiconductor-Metal (MSM) diode, and a varistor.
FIGS. 39A and 39B are graphs schematically plotting current-voltage characteristics of a current steering element. FIG. 39A shows current-voltage characteristics of a bipolar current steering element such as a MIM diode, a MSM diode, or a varistor. FIG. 39B shows current-voltage characteristics of a Schottky diode.
As shown in FIG. 39B, a Schottky diode shows non-linear electrical resistance characteristics, but current-voltage characteristics regarding respective polarities of applied voltage are not symmetrical at all.
On the other hand, as shown in FIG. 39A, a two-terminal element, such as a MIM diode, a MSM diode, or a varistor, shows non-linear electrical resistance characteristics, and current-voltage characteristics regarding respective polarities of applied voltage are actually symmetrical. More specifically, the two-terminal element has characteristics in which current variation caused by positive voltage application and current variation caused by negative voltage application are symmetrical with respect to an origin 0. Moreover, the two-terminal element has very high electrical resistance in a voltage range (namely, range C) where an applied voltage is equal to or lower than a first critical voltage (lower-limit voltage in a range A) and equal to or higher than a second critical voltage (upper-limit voltage in a range B). On the other hand, if the applied voltage exceeds the first critical voltage or is lower than the second critical voltage, the electrical resistance of the two-terminal element is rapidly decreased. More specifically, the two-terminal element has non-linear electrical resistance characteristics in which large current flows through a selected variable resistance element when the applied voltage exceeds the first critical voltage or is lower than the second critical voltage.
Therefore, if such two-terminal elements are used as bipolar current steering elements, it is possible to prevent write didturb in a crosspoint nonvolatile memory including bipolar variable resistance elements capable of performing high-speed operation both in the setting process and in the resetting process.
Meanwhile, in the variable resistance memory, when data is to be written to a target variable resistance element, the variable resistance element is pulsed to change its electrical resistance value. Therefore, the variable resistance element becomes in a high resistance state or in a low resistance state. In general, it is necessary to allow relatively large current to flow through the variable resistance element to change the resistance state, although the current mount depends heavily on a material, structure, and the like of the variable resistance element. For example, regarding processing performed in a memory including variable resistance elements, there has been disclosed a technology of using a varistor to allow current of a current density of 30000 A/cm2 or more to flow through variable resistance element to write data into the variable resistance element (see Patent Literature 2, for example). In recent years, various examinations have been conducted to reduce current required to write data into a variable resistance element. As a result, it is presently considered that a current density of current required to write data into a variable resistance element is not always 30000 A/cm2 or more. However, it is common that a relatively large current in a range from 10000 A/cm2 to tens of thousand of A/cm2 is required to the data writing.